This invention relates to a process for introducing mechanical vibrations into vessels through a flexible membrane installed in the vessel wall by means of a push rod and to an arrangement for introducing vibrations produced by a vibration generator into a vessel, the vibrations being designed to be delivered through a linkage as vibration pickup to a flexible membrane installed in the vessel wall.
In many chemical reactions between immiscible phases, it is important to obtain a good mass transfer between the phases. Thus, reactions between liquids and the gases dispersed therein take place at the phase interface. The overall reaction rate depends on the transfer of the gas into the liquid phase. In many cases, this transfer of the gas limits the reaction rate.
There are known processes which accelerate the transfer of a gas into a liquid and hence reduce energy costs or increase the specific volume yield or both. Thus, it has been proposed to increase the reaction rate in a process for hardening unsaturated fats by catalytic hydrogenation with hydrogen by exposing the reaction mixture to high-energy ultrasound. High increases in the reaction rate were measured but unfortunately cannot be attributed to an improvement in mass transport. Instead, detailed investigations have shown that they are based on an increase in the temperature of the reaction mixture through the effect of the high-energy ultrasound waves. Comparative measurements with and without exposure to high-energy ultrasound under isothermal conditions revealed no differences in the overall reaction rate.
Other disadvantages are an obstacle to the application of this proposal for industrial purposes. The acoustic power density used in this process amounts to more than 100 W/I reactor volume and is thus far too high for industrial purposes. Also, ultrasonic waves are very heavily attenuated in gas/liquid dispersions so that ultrasonication is uneconomical for relatively large reactors suitable for industrial purposes.
In an Article in Trans. Instn. Chem. Engnrs. 44 (1966) T91, G. J. Jameson reports on another process for increasing mass transfer in gas/liquid dispersions by sound waves. In this process, a gas/liquid column is made to resonate through low-frequency sound. This process is unsuitable for industrial purposes because very low frequencies would have to be used for relatively large vessels and hence tall liquid/gas columns. These frequencies would have to be below 10 Hz and would have to have amplitudes of more than 0.5 m in order significantly to improve mass transport.
DE 44 36 064 A1 describes a process in which the frequency of the sound acting on the gas/liquid column is substantially equal to one of the resonance frequencies of the phase interface between the gas bubbles and the liquid. The power density of the sound is below the levels sufficient for degassing the liquid. Under these conditions, mass transfer in the gas/liquid mixture is clearly improved and the overall reaction rate in a liquid/gas reaction is greatly increased.
In such a process, the sound waves can be introduced into a reactor or other vessel through the delivery of the vibrations produced by a vibration generator via a linkage to a flexible membrane. This membrane is installed in the wall of the reactor or vessel. However, this is only possible when substantially the same pressure prevails on both sides of the membrane. Accordingly, arrangements in which ambient pressure prevails in the vessel or reactor belong to the prior art. This technique cannot be applied when the difference in pressure between the two sides of the membrane is such that the membrane is significantly moved from its neutral or rest position. Thus, where a pressure of 20 bar prevails in the vessel, a compressive force of about 2 tonnes is applied, for example, to a relatively small membrane area of 100 cm.sup.2.